Power supply and signal conditioner for electroner for electronic instrumentation

ABSTRACT

An electronic signal conditioner for use with process control instruments is described in which a signal from a transmitting instrument is received and conditioned by way of damping and current limiting before being retransmitted to control apparatus. The output signal is essentially independent of the internal impedance of the receiving apparatus.

O I United States Patent 11113, 7

[72] Inventor David E. Nelson [56] References Cited 2 m 9 UNITED STATESPATENTS [211 P 8 3,231,829 1/1966 Reid 331/27 x [221' Ned My 3 440 51s4/1969 c1111 ard m1 323/4 [45] Patented Oct. 19,1971 54481372 6/1969 f323/39) [73] Assignee DeZurilt Corporation 1 Sartell, Minn. PrimaryExaminer- Roy Lake Assistant Examinr.lames B. Mullins Attorney-Wolfe,Hubbard, Leydig, Voit & Osann [54] POWER SUPPLY AND SIGNAL CONDITIONERFOR ELECTRONER FOR ELECTRONIC INSTRUMENTATION 8 4 Damn: ABSTRACT: Anelectronic signal conditioner for use with [52] US. Cl 307/297, processcontrol instruments is described in which a signal from 323/4, 323/39 atransmitting instrument is received and conditioned by way [51] Int. Cl"03k 1/14 of damping and current limiting before being retransmitted to[50] 307/297; control apparatus. The output signal is essentiallyindependent 323/4, 39, 22 T of the internal impedance of the receivingapparatus.

g /0 I I l (U/VJ/f/Z/Kfy (WI/ '06 Willi/7' [e Wf/WA/[A/I'J l (fille /V76764/4! lie/I! A 4/41/72? mar/[e Fifi/47016 I 1 i [7 11 ZZ 11 EZ/ I W mnze 7 Vi/V44 1, CL wa /2y (wt/flame /J I /7 POWER SUPPLY AND SIGNALCONDITIONER FOR ELECTRONER FOR ELECTRONIC INSTRUMENTATION DESCRIPTION OFTHE INVENTION This invention relates generally to electronicinstrumentation, and in its principal aspect concerns an improved meansfor receiving and conditioning an electrical signal and retransmittingthe signal in improved form to further instrumentation or to processcontrol apparatus.

A principal objectof the present invention is to provide apparatus forreceiving, conditioning and retransmitting an electrical signal from adistant transmitting device to subsequent control or instrumentationequipment. As part of the conditioning function, it is intended toprovide current-limiting means for protection of overload, damping meansfor damping of noise signals including rapid fluctuations andtransients, and regulation of the output signal in a manner which isessentially independent of the load impedance of the instrumentationorcontrol apparatus into which it is fed.

More specifically, it is an object of the present invention to provide asignal conditioning apparatus in which the output signal is proportionalto any steady-state input signal, but with a damping function thatcauses the rate of change of the output signal to be constant regardlessof the magnitude of input signal change, in contrast to existingresistance-capacitance damping circuits which cause the signal toapproach its steady-state valve asymptotically. With the improvedconditioning apparatus of the present invention, the transmitted outputsignal is enabled to follow the input signal with substantially linearresponse until reaching its new steady-state value.

These and other objects and advantages of the invention will becomeapparent upon reading the following specification and upon reference tothe drawings, in which:

FIG: 1 is a simplified schematic block diagram of a power supply andsignal-conditioning apparatus exemplifying the present invention;

FIG. 2 is a graph of typical response curves obtained from aconditioning circuit constructed according to the invention for astep-function fluctuation (solid lines) and a pulse fluctuation (dashedlines), as compared with the corresponding response curves typical of aprior art resistance-capacitance circuit capable of reachingsubstantially the same steady-state value e at a time t,

FIG. 3 is a graph of typical response curves similar to FIG. 2, for twostep-function input signal fluctuations of different magnitudes, againas compared with the corresponding response curves typical of a priorart resistance-capacitance circuit capable of reaching substantially thesame steady-state valves e and e, at a time 1,; and

FIG. 4 is a detailed schematic circuit diagram of the power supply andsignal conditioner shown in FIG. 1.

BACKGROUND AND ENVIRONMENT The requirements of electronic compatibilityin process control equipment have become more and more strict asindustrial processes become increasingly more automated. A flow-sensingdevice, for example, may have an electrical output characteristic whichis not directly compatible with the instrumentation which depends on itfor information. The output signal from the transmitter of the sensingdevice may be of the wrong magnitude or range, may have the wrongelectrical impedance for proper matching, and may be subject to sharpsignal fluctuations and transients as well as spikes of excessivevoltage or current which can disrupt or damage the receiving equipment.Such equipment often includes computers for automated process controlapplications and the consequences of such disruption or damage aresubstantial. An entire industrial process can often be shut down by asingle such malfunction. The compatibility problem is in part caused bythe common use of four different signal level ranges for instrumentationequipment. These ranges are nominally 1-5 ma., 4-20 ma., and l-5 volts.Unless sensing equipment is carefully chosen to be compatible with theinstrumentation with which it is to be used, the system will notfunction.

Another problem of compatibility arises from the presence of sharpfluctuations and transients, including random variations described assignal noise. Some sensing equipment, particularly sensors of smallphysical size, will exhibit enhanced high-frequency responsecharacteristics and will detect and transmit rapid local fluctuationseven though its related process control equipment may be incapable ofresponding rapidly enough to follow. It is unnecessary and oftendetrimental for the process control equipment to even attempt tofollow'such small and random variations, particularly where the presenceof feedback in the system may give rise to oscillation and instability.In addition, a control system which is proportionally responsive to therate of increase detected by the sensor will overreact to a sharp pulseor spike even of momentary duration, again causing needless changes inthe system which must be corrected for when the pulse or spike isdissipated.

Finally, certain processes are inherently subject to rapid and randomlocalized fluctuations rather than changing smoothly and gradually. Forexample, the flow of wood pulp in a conduit may be detected by a sensorof relatively small physical size which can be momentarily deflected bylocalized pockets of heavier material even though the average density ofthe material is essentially constant. Since these local variationscannot be controlled, they must be classified as noise signals and canbe detrimental to the control function.

Some solutions have been proposed in which damping is provided withinthe system, either electronically or mechanically. Mechanical devicescan be installed in the sensor in the form of dashpots and the like butsuch mechanical devices are inherently difficult to adjust, subject tochanging characteristics with age, temperature and wear, and may not beresponsive to very small input fluctuations. Electronic damping has beenused with simple amplifier circuits but such damping has generally beenof the type which acts directly on the input or output signal with asimple resistance-capacitance damping circuit. A difiiculty with suchdamping circuits is that they are not practical with circuits havingrelatively low impedances. The capacitance necessary to furnisheffective damping with an impedance of, for example, ohms, may beseveral thousand microfarads. Capacitors of this size are subject todeterioration with age and temperature and the damping function willconsequently change with time. In addition, these capacitors areexpensive to buy, large to install, and may differ widely in capacitancefor the same nominal value.

If the output rangeof the signal conditioning device is to be madevariable, this often requires extensive changes in the damping circuitto achieve the same relative damping effectiveness and constitutesanother disadvantage in the damping circuits of existing equipment.

GENERAL OPERATION Turning first to FIG. 1, the simplified block diagramshown therein illustrates the various functional elements of theimproved apparatus constructed according to the invention. In thisillustrative embodiment, the apparatus combines the functions of a powersupply and signal conditioner for a process sensor such as a wood pulpconsistency transmitter 10 and is mounted on a single chassis (notshown) to which control instruments 11 or other related equipment may besimply and directly connected. The consistency transmitter 10 or othersensing device is connected to one set of terminals and the controlinstrumentation 11, whether it be a meter or a sophisticated computerapparatus, is connected at a second set of terminals on the samechassis.

An internal power supply I2 is connected to a source of power such as anordinary AC line L,, L, and furnishes electrical power for the rest ofthe apparatus. Incoming line voltage is rectified and filtered by thepower supply 12 to produce DC power for the transmitter and theremaining portion of the current conditioning apparatus including theoutput loop. The DC current going to the transmitter 10 is impressedwith a signal which is a function of the process being monitored andthen returns to the signal-conditioning apparatus. This output signalfrom the transmitter goes first through a receiving means includingcurrent-limiting circuit 13 which protects the remaining portions of thecircuit from overload and then through a sensing resistor 15 to generatea signal voltage at a point 16. This voltage is then supplied to acomparison means consisting of a high-impedance comparator circuit 17.

The comparator circuit 17 compares the signal produced at point 16 withthat produced at an adjoining point 18, which is a function of outputcurrent passing through a second sensing resistor 20. The instantaneousmagnitude of the output current to the instrumentation 11 is controlledby a regulator circuit 21, and its magnitude is represented at all timesby the voltage appearing at point 18 due to the voltage drop across thesecond sensing resistor 20. If there is no change in the steady-statesignals produced by the transmitter 10 and seen by the instrumentation11, then no error signal is produced by the comparator circuit 17. If,however, a fluctuation appears, then an error signal is produced by thecomparator circuit 17 and supplied as a correction to the regulator 21through a damping means consisting of a signal-damping circuit 22.

As a principal feature of the invention, increased accuracy anddependability are achieved because the damping circuit 22 is separatefrom the output signal path. With conventional circuits, a change indamping capacitors will have the same effect on the instrumentation 11as a change in input signal, resulting in an error signal which isuncorrected. While this can be minimized by adding feedback circuitry,the improved circuit is free from this requirement and can therefore belocated at any convenient point in the signal transmission line. Theindependence of the damping circuit 22 from both input and output signalpaths also allows it to use a desirably high impedance for ease ofconstruction and increase component reliability. The error signal afterdamping by the damping circuit 22 causes a corrective change in theregulator circuit 21 to vary the current flow through theinstrumentation 11 and the second sensing resistor 20, bringing thevoltage at point 18 back into correspondence with that at point 16.

It will be seen that because the comparator circuit 17 is sensitive tothe voltage at point 18 and not the absolute value of instrumentcurrent, the current flowing through the output loop consisting of theinstrumentation 11, regulator 21 and second sensing resistor may bevaried by simply changing the value of the second sensing resistor 20.Different ranges of output current may be obtained in this way withoutaffecting the operation of the comparator circuit 17 or the dampingcircuit 22, making them entirely independent of the output range in use.

CIRCUIT DESCRIPTION A detailed understanding of the operation of thecircuit of the present exemplary embodiment of the invention may begained by referring to H6. 4. The power supply 12 receives AC powerthrough a transformer 23 and the voltage is rectified by a diode bridge24. A filtering circuit consisting of a resistor 25 and filtercapacitors 26 provides a regulated DC output to both the transmitter 10and the instrumentation 11.

From the transmitter 10, an input signal is obtained which is directedto the current-limiting circuit 13. A transistor 27 is normally held inan on condition by a resistor 28 and presents very little impedance tothe signal current. If the input signal becomes excessive, at somepredetermined point, preferably 1 10 percent of the rated signalcurrent, the base voltage of the transistor 27 will rise to a point thatwill cause current to flow through a blocking diode 29 into a zenerdiode 30, preventing the base voltage of the transistor 27 from risingany further. Any further increases in signal current will then only tendto turn the transistor 27 off, thereby limiting the peak voltage atpoint 16 and protecting the remainder of the circuit.

Assuming normal operation and a transmitter 10 having a characteristiccurrent range of 10-50 ma., a signal value of 50 percent will representan input current of 30 ma. This current flowing through the sensingresistor 15 develops an input signal at the point 16 which is fed to thecomparator circuit 17 containing transistors 31 and 32. The totalcurrent through the transistors 31, 32 is maintained constant by acontrol transistor 33. The two transistors 31, 32 are each connected toa respective resistor 35, 36 to form parallel legs of a bridge circuit.At the center of the bridge is an error detection transistor 37 which,when the bridge is balanced, will conduct within its proportional rangeand maintain a constant voltage at the regulating circuit 21 in thepresent example of about 4.5 volts as detected at point 38 in thedamping circuit 22.

The damping circuit consists of a resistor 40 and capacitors 41, 42, 43.Since the damping circuit is independent it may desirably operate athigh impedance. The resistor 40 has a relatively high value such as 750kilohms and the capacitors 41, 42, 43 are relatively low valuesdepending on the degree of damping required, being 2, 50 and 225microfarad capacitors in the present example. By leaving all thecapacitors 41,42, 43 in parallel a maximum damping effect is obtainedwith a response time (with a step-function input) ranging up toapproximately 250 seconds. If all but the 2-microfarad capacitor areremoved from the circuit, the response time is reduced to approximately2 seconds.

The error signal developed by the transistor 37 and applied to theregulator circuit 21 regulates the output current from theinstrumentation 11 through the regulating transistors 45, 46. The outputcurrent is conducted through the second sensing resistor 20 whichdevelops a voltage at point 18 corresponding to that developed by theinput current at point 16, and the voltage at point 18 is applied to thetransistor 32. If the voltages at points 16 and 18 are the same thecurrents through both legs of the bridge formed by the transistors 31,32 and resistors 35, 36 are the same and there is no change in thecurrent in the error-detecting transistor 37, which therefore remains inits proportional range.

If, however, the input signal should increase from 50 percent to 75percent, for example, reflecting a change in input current from 30 to 40ma., conduction through the transistor 31 increases, dropping the basevoltage of the error detection transistor 37 and driving it tosaturation. The saturation current from the transistor 37 causes point47 to reach approximately ll volts in the present example, therebycausing the damping capacitors 41, 42, 43 to begin charging throughresistor 40 and increasing the voltage at point 38, thus increasing thevoltage applied to the regulator circuit 21 as well. With the examplegiven, the voltage at point 38 will rise at a sub- .stantially constantrate as determined by the charging of the capacitors 41, 42, 43 from therelatively high voltage of ll volts at point 47 until the voltage at thesecond sensing resistor 20 and the point 18 becomes substantially thesame as that at point 16, thereby terminating the saturated conductioncondition of error detection transistor 37 and stopping the charging ofthe capacitors 41, 42, 43 quite abruptly. At this point the system isagain in equilibrium with an output current corresponding to the 40 ma.input signal current. The output signal has been increased at a roughlyconstant rate equal to the charging rate of the damping capacitors 41,42, 43 through the resistor 40.

The effect of this mode of operation is shown in F IG. 3. Instead of theconventional asymptotic exponential resistancecapacitance charging curveof prior art apparatus, the rate of change is essentially independent ofthe magnitude of the input fluctuation to be damped, and therefore thechange takes place in a substantially linear manner until the newequilibrium point is reached and the rate of change stops abruptly. Thechange in the output current is governed by a resistance-capacitancecharacteristic representative of a fluctuation greatly larger than theactual change to be accommodated, and the charging curve corresponds tothe initial portion of this characteristic. However, because the errordetection transistor 37 is abruptly cut off when the required correctionhas been applied, the correction stops at this point and the system isagain in balance.

For a decreasing signal input, the response characteristic is similar. Adecreasing input signal sensed by the sensing resistor causes thetransistor 31 to conduct less than the transistor 32, unbalancing thebridge in the opposite direction. This change in polarity shifts theerror detection transistor 37 from its equilibrium state in theproportional range into sharp cutoff, allowing the damping capacitors41, 42, 43 to discharge through the resistor 40 until the voltage atpoint 38 drops sufficiently to cause a corresponding change in theoutput current through the regulator circuit 21. The error detectiontransistor 37 remains in cutoff until the voltage at point 18 againcorresponds to the newly reduced voltage at point 16, which againbalances the bridge and brings the error detection transistor 37 backinto its normal proportional condition.

As previously noted, by simple changing the value of the second sensingresistor 20 the output current range may be easily changed withoutinterfering with the operation of the current-limiting circuit 13,thecomparison circuit 17 or the damping circuit 22. By removing variousones of the damping capacitors 41, 42, 43 the damping function may alsobe easily varied. With the lowest capacitance valve the outputessentially follows the input, but with increasing capacitance greaterdamping efi'ect is provided up to the maximum available for extremedamping, as may be necessary in applications in which the sensingtransmitter 10 is subject to large amounts of signal noise and abrupttransient conditions. Changing the value of the capacitance in thedamping circuit 22 changes the time constant of theresistance-capacitance network therein, but as provided for by theinvention the usual exponential time constant is characteristic of themuch larger change than is actually accommodated, allowing thecorrection curve to be ap proximately linear within the actual range ofcompensation. When the comparison circuit 17 detects the desiredcorrespondence of input and output signals, the correction functioninput is abruptly terminated at the new operating point.

As a result of the invention the output signal response to a pulse orstep-function input is nearly constant, which is a desirable goal in allapplications of process control. The effect of having different responsecurves for different rates of load change is eliminated, and controlsystem which are tuned" to handle relatively slow load changes will notbecome unstable should faster load changes occur. The characteristic ofprior art systems which associates a slow rate of load change with asmall magnitude change and a fast rate of change with a large magnitudechange is eliminated. The rate of output signal change is essentiallyfixed regardless of the size of the rate of input change. It istherefore possible to tune the rest of the control system for optimumoperation without regard to the magnitude of the input changes whichmust be handled. This feature is of particular advantage when handling asystem startup on automatic control, which generally presents thelargest load changes encountered during system operations.

The following is claimed as invention:

1. An electronic signal conditioner for conditioning electrical signalspassing from a transmitter to control instruments,

,said conditioner comprising the combination of first sensing means forsensing the amplitude of electrical signals from said transmitter,second sensing means for sensing the amplitude of electrical signalspassing through said control instruments, regulator means operativelyassociated with said control instruments for regulating the amplitude ofthe electrical signals passing through said instruments, an electricalcomparator operatively connected to said first and second sensing meansfor producing an error signal in response to the predetermineddifference between the amplitude of the signals sensed by said first andsecond sensing means, said comparator being operatively connected tosaid regulator means for supplying said error signal to said regulatormeans, and electrical signaldamping means operatively connected to saidcomparator and said regulator means for damping said error signal.

2. Apparatus as defined in claim 1 in which the first sensing meansincludes current-sensitive means for detecting input signal currentabove a predeterrmned level and llmrtmg means for bypassing current inexcess of said level around said comparator.

3. Apparatus as defined in claim 1 wherein the comparator includes abridge circuit having a first leg including means connected to the firstsensing means for producing a leg current in said first leg which is afunction of the signal from the transmitter, and a second leg, parallelto said first leg and including means connected to the second sensingmeans for producing a leg current in said second leg which is a functionof the signals passing through the control instruments, whereby theerror signal produced is responsive to difference between the currentsin the respective legs.

4. Apparatus as defined in claim 3 including a transistor connectedbetween the center points of the respective parallel legs of the bridgecircuit so as to conduct within its proportional range during conditionsof bridge balance when no signal appears between the respective centerpoints of the parallel legs, said transistor being driven to asaturation condition by substantial variations of one polarity in thesignal from the transmitter and to a current cutoff condition forsubstantial variations of opposite polarity in the signal from thetransmitter.

5. Apparatus as defined in claim 1 wherein said damping means includes aresistance-capacitance network, whereby the time constant for thedamping function is essentially inde pendent of the absolute magnitudeof input signal variation over the normal range of operation.

6. An electronic signal-conditioning method for conditioning electricalsignals passing from a transmitter to control instruments, saidconditioning method comprising the steps of sensing the amplitude offirst electrical signals from said transmitter, sensing the amplitude ofsecond electrical signals passing through said control instruments,comparing the respective amplitudes of said first and second electricalsignals and producing an error signal in response to a predetermineddifference between the amplitudes of the compared signals, damping saiderror signal, and regulating the amplitude of said second signal inaccordance with the damped error signal.

7. An electronic signal-conditioning method as set forth in claim 6wherein the respective amplitudes of said first and second electricalsignals are sensed across first and second sensing resistors, and therange of the second signals relative to the first signal is changed bychanging the value of the second sensing resistor.

8. An electronic signal conditioning method as set forth-in claim 6wherein said damping is effected by a resistancecapacitance network, andthe damping level is changed by adjusting the capacitance values in saidnetwork.

1. An electronic signal conditioner for conditioning electrical signalspassing from a transmitter to control instruments, said conditionercomprising the combination of first sensing means for sensing theamplitude of electrical signals from said transmitter, second sensingmeans for sensing the amplitude of electrical signals passing throughsaid control instruments, regulator means operatively associated withsaid control instruments for regulating the amplitude of the electricalsignals passing through said instruments, an electrical comparatoroperatively connected to said first and second sensing means forproducing an error signal in response to the predetermined differencebetween the amplitude of the signals sensed by said first and secondsensing means, said comparator being operatively connected to saidregulator means for supplying said error signal to said regulator means,and electrical signaldamping means operatively connected to saidcomparator and said regulator means for damping said error signal. 2.Apparatus as defined in claim 1 in which the first sensing meansincludes current-sensitive means for detecting input signal currentabove a predetermined level and limiting means for bypassing current inexcess of said level around said comparator.
 3. Apparatus as defined inclaim 1 wherein the comparator includes a bridge circuit having a firstleg including means connected to the first sensing means for producing aleg current in said first leg which is a function of the signal from thetransmitter, and a second leg, parallel to said first leg and includingmeans connected to the second sensing means for producing a leg currentin said second leg which is a function of the signals passing throughthe control instruments, whereby the error signal produced is responsiveto difference between the currents in the respective legs.
 4. Apparatusas defined in claim 3 including a transistor connected between thecenter points of the respective parallel legs of the bridge circuit soas to conduct within its proportional range during conditions of bridgebalance when no signal appears between the respective center points ofthe parallel legs, said transistor being driven to a saturationcondition by substantial variations of one polarity in the signal fromthe transmitter and to a current cutoff condition for substantialvariations of opposite polarity in the signal from the transmitter. 5.Apparatus as defined in claim 1 wherein said damping means includes aresistance-capacitance network, whereby the time constant for thedamping function is essentially independent of the absolute magnitude ofinput signal variation over the normal range of operation.
 6. Anelectronic signal-conditioning method for conditioning electricalsignals passing from a transmitter to control instruments, saidconditioning method cOmprising the steps of sensing the amplitude offirst electrical signals from said transmitter, sensing the amplitude ofsecond electrical signals passing through said control instruments,comparing the respective amplitudes of said first and second electricalsignals and producing an error signal in response to a predetermineddifference between the amplitudes of the compared signals, damping saiderror signal, and regulating the amplitude of said second signal inaccordance with the damped error signal.
 7. An electronicsignal-conditioning method as set forth in claim 6 wherein therespective amplitudes of said first and second electrical signals aresensed across first and second sensing resistors, and the range of thesecond signals relative to the first signal is changed by changing thevalue of the second sensing resistor.
 8. An electronic signalconditioning method as set forth in claim 6 wherein said damping iseffected by a resistance-capacitance network, and the damping level ischanged by adjusting the capacitance values in said network.